Experiment on salt exchange between sediments and ponding water in drainage ditches of saline farmland

Abstract: In arid and semi-arid agricultural regions, artificial drainage is provided to maintain salt balance of the saline farmland. As a result of poor drainage outlet in some sites, salt accumulation in the drainage ditches often leads to salinity rising to a critical level that threatens ecological functions of the ditch system. Periodic flushing with fresh water has been suggested to slow down the process of salinity growth in such ditch system. But the effect of the slow moving freshwater in drainage ditches on salt balance is unclear. In order to investigate the internal mechanism of salt release in saline drainage ditches when freshwater is added, and to examine the relationship of salt exchange between sediment and the overlying water under hydrostatic condition, we conducted a laboratory study to measure spatial and temporal variations of salinity in sediment and water interface with two experimental plexiglass columns. Each column is 15 cm in (inner) diameter and 100 cm high. The column has a top cover to prevent the evaporation loss of water. Sediments were filled to 30 cm thick at the bottom and covered by a freshwater layer of 65 cm. Room temperature was kept at (15±1)℃ during the experiment. The sediment salinity was measured by extracting the pore water through four sampling holes on each column wall. Electrical conductivity of the sediment pore water and the overlying water were measured every 24 hours during the monitoring period. The sediments were taken from a saline agricultural drainage ditches in Lubotan reclamation area in Shaanxi, China, where salinity elevation in the drainage ditches presents a problem to the ecological functions of the ditch system, and freshwater is available for salt flushing in the end of the irrigation season. We measured salinity variations in water and sediments in the experimental columns continuously for a period of 648 hours, then calculated salt fluxes in the water and sediment interface based on measured sediment properties. The monitoring data showed that there existed a linear salinity variation zone (or the diffusion boundary layer) within 10 cm of the sediment and water interface, salinity of water remained stable above 10 cm. The results also showed that salt diffusion within the boundary layer slowed the salt release from the sediment, and the slowing effect increased with the thickness of the boundary layer. Flux calculation results showed that salt fluxes released from the sediment can be predicted with a power function, i.e., salt fluxes decreased rapidly initially and then stabilized with time. Observed salinity change in sediment indicates that salt release through molecular diffusion only in drainage ditches is a slow process, which may have only limited impact on salt balance in drainage ditches when the flushing water moves slowly. And hydrodynamic dispersion is required to speed up release of salt from the ditch sediment in order to remove accumulated salts in the drainage ditches in saline environment.